I think popular wisdom is "look at the course map." The speed you want to carry through a slalom will vary with the type of slalom.
If it is a decerasing slalom (cones get closer) - you should gradually ease off of the power. The key is **smoooooth*** and avoiding the brakes. Brakes = cones.
If it is an increasing slalom (I hate typing that word) - you should enter with less speed and gradually roll onto the power. Again, the key will be smooth and to know what comes at the end of the slalom (straight segment, tight turn, etc.). That will dictate how much risk adding power is worth. Personally, in my autox days, I found that repeated transition set my poorly prepared car off enough that just maintaining throttle was a chore.
If it is a constant slalom, keep consistent throttle input.
Of course, with any slalom, the most important thing is vision and timing. Keep your eyes up and don't get behind. Unfortunately, I don't have as much autox experience with the 240 as some, so I never perfected the art. I'm sure others will have some specific tips.